Implantable multi-channel electrodes for neuro-stimulation or neuro-modulation need to be mechanically robust, and yet flexible and of small size to be inserted into body cavities such as the human cochlea, or to be inserted into a body organ such as the brain. Typically, the wires in most implant electrodes have a homogenous shape from one end to the other: either generally straight, repeating coiled loops, or recurring wave shapes. In environments where the implanted electrodes continuously move relative to the surrounding tissues, matching the mechanical properties of the electrodes to the properties of the surrounding tissues is important for avoiding adverse biological reactions and massive scar tissue generation.
Implant electrodes are being developed for insertion ever more deeply into body cavities of progressively more complex shape. So an implant electrode should have non-uniform and non-homogeneous mechanical properties (e.g., bending and flexing) to accommodate the tortuous path that it must take, and also for maintaining biological compatibility with the surrounding tissue. There may be some parts of an implant electrode that need to be highly resistant to micro-movement (e.g., the portion of a cochlear implant electrode which lies immediately under the skin on the skull). Other portions of the implant electrode may need to be very bendable to accommodate a convoluted insertion path (e.g., the portion of a cochlear implant electrode that goes into the cochlea). Some portions of the implant electrode may be exposed to occasional impact force and so may need to be very resistant to external impact (e.g., portions of a cochlear implant electrode under the skin on the skull).
Some compromise in these factors must be achieved in circumstances where high flexibility is needed but space is very limited (e.g. as in the cochlea). Electrode structures that are highly resistant to micro-movements tend to occupy relatively more space, whereas electrode structures that are small in size tend to be relatively rigid. Presently, as the number of electrode stimulation channels increases, the number of corresponding metallic wires in the electrodes also increases. That in turn causes the implant electrodes to become increasingly rigid.
As used herein, the term “electrode array” refers to the apical end section of the implant electrode that penetrates into a cochlea scala of the inner ear. An electrode array has multiple electrode contacts on or slightly recessed below its outer surface for applying one or more electrical stimulation signals to target audio neural tissue. An “electrode lead” refers to the basal portion of the implant electrode that goes from the implant housing to the electrode array. It usually has no contacts except perhaps a ground electrode and it encloses connecting wires delivering the electrical stimulation signals to the electrode contacts on the electrode array. The term “electrode” refers to the entire implant electrode from end to end, that is, the combination of the electrode array and the electrode lead.